Human Evolution

Animal Sciences
COPYRIGHT 2002 The Gale Group Inc.

Human Evolution

Human evolution is the lengthy process of change by which people originated from apelike ancestors starting nearly five million years ago. The modern scientific study of human evolution is called paleoanthropology. A subfield of anthropology, this discipline searches for the roots of human physical traits, culture, and behavior. It attempts to answer questions: What makes us human? When and why did we begin to walk upright? How did our brains, language, art, music, and religion develop? By approaching these questions from a variety of directions, using information learned from other disciplines such as molecular biology, paleontology, archaeology, sociology, and biology, we continue to increase knowledge of our evolutionary origins.

Most cultures throughout human history have myths, stories, and ideas about how life and culture came into existence. Although the current theory of evolution, based on the ideas of Charles Darwin, is accepted by a majority of scientists in our time, it is important to remember that many earlier ideas were recognized as well.

Darwin's books, On the Origins of the Species by Natural Selection (1859) and The Descent of Man (1872), expressed his theory of evolution and revolutionized the study of life and human origins.

Darwin presented evidence showing that natural species including humans have changed, or evolved, over long spans of time. He also argued that radically new forms of life develop from existing species. He noted that all organisms compete with one another for food, space, mates, and other things needed for survival and reproduction. The most successful individuals in this competition have the greatest chance of reproducing and passing these characteristics on to offspring. Over hundreds of thousands of generations, one form of life can evolve into one or more other forms. Darwin called this process natural selection.

Modern science now understands that the mechanism for evolutionary change resides in genes , the basic building block of heredity . Genes determine how the body, and often the behavior, of an organism will develop over the course of its life. Certain information in genes can change, and over time this genetic change can actually alter a species' overall way of life.

In recent decades, biological and social scientists have made impressive strides in understanding our complex physical and cultural origins. Their research has revealed gradual alterations in our genetic structure, as well as shifts in culture and behavior, that have transformed humankind into the planet-dominant species.

Scientists estimate that our human ancestors began to diverge from the African primates between eight million and five million years ago. This figure is the result of studying the genetic makeup of humans and apes, and then calculating approximately how long it took for those differences to develop. Using similar methods of comparing genetic variation among human populations around the world, it is thought that all people living today share a common genetic ancestor.

Early Life in Africa

The human story begins in one of the most geologically fascinating areas on Earth, the Great Rift Valley of Africa. It is an enormous split torn into Earth's crust that runs from the forests in Tanzania to the deserts of Ethiopia. In some places the rift is thousands of feet deep and exposes the last fifteen million years of the earth's history. Here, fossil remains of our earliest ancestors can be found. Humankind appears to have first evolved in Africa, and the fossils of early humans, or hominids, who lived between five million and two million years ago, come entirely from Africa.

Starting with the modern human skull, it is possible to trace our ancestry back millions of years. As we travel back in time, our ancestors look less and less like us and begin to resemble our closest relatives, the African apes. Because our physical and genetic characteristics are similar, evolutionary theory offers evidence that ancestral humans had a very close relationship to a group of primates, the apes. Humans, chimpanzees, gorillas, and the large apes of Africa share a common ancestor that lived between eight million and five million years ago.

Humans, or hominids, belong to the scientific order named Primates, a group of more than 230 species of animals that includes the monkeys, lemurs, and apes. Modern humans have a number of physical characteristics resembling our ape ancestry. The social systems of humans also share similarities with the African apes and other primates, such as baboons, chimpanzees, and rhesus monkeys. Chimps live, groom, feed, and hunt together and form strong family bonds. Early humankind probably had a similar lifestyle.

Scientists now know that nearly 98 percent of the genes in humans and chimpanzees are identical, making chimps the closest living biological relative of humans. However, there are fundamental differences between modern humans and their primate relatives. The human brain is larger and more complex, giving humankind the ability to communicate through language, art, and symbols, to walk upright, and to develop a throat structure that makes speech possible.

One of the earliest defining human traits is bipedalism, the ability to walk upright on two legs. This characteristic evolved over four million years ago. Other important human characteristics, such as a large and complex brain, the ability to make and use tools, and the capacity for language and culture, developed more recently. Many of what we consider advanced traits, such as art, religion, and different expressions of cultural diversity, emerged during the past 100,000 years.

Most paleoanthropologists today recognize ten to fifteen different groups of early humans. They do not agree, however, about how they are related or which ones simply died out along the way. Researchers also disagree about how to describe, identify, and classify these early human species, and what factors influenced the evolution and extinction of each species.

Early Humans: Evolution of Australopithecines

Nearly five million years ago in Africa, an apelike species evolved with two important traits that distinguished it from the apes. This species had small canine teeth (next to the four front teeth), and it was bipedal , meaning it could walk on two legs instead of four. Scientists refer to these earliest human species as australopithecines, or australopith for short.

The fossil record shows that there is not an orderly sequence leading from one form to another. Several groups lived at the same time and characteristics developed at different rates; therefore the human family tree suggests a long and complex past.

Fossils from several early australopith species that lived between four million and two million years ago clearly demonstrate a variety of adaptations that mark the transition between ape to human. Prior to four million years ago, fossil remains are scarce and incomplete; where available, however, they do show a primitive combination of ape and human features.

Most of the key characteristics that stand out as distinctly human are related to their bipedal stance. The australopiths had an S-shaped spine that allowed for balance when standing. The opening through which the spinal cord attached to the brain was positioned more forward, allowing for the head to be balanced over the upright spine. The pelvic bone was shorter and broader than in apes, giving the pelvis a bowl shape that supported the internal organs when standing or walking upright. The upper legs angled inward allowing the knees to support the body while standing or walking. Shorter and less flexible toes functioned as rigid levers for pushing off the ground with each step.

Most early species had small canine teeth, a projecting face, and a small brain. They weighed between 22 and 37 kilograms (60 to 100 pounds), and were 0.9 to 1.5 meters (3 to 5 feet) tall. Males were generally larger than females. Both had curved fingers and long thumbs with a wide range of movement. The apes, in comparison, have longer, more curved, and stronger fingers that make them well adapted for hanging and swinging from branches. Apes also have short thumbs, which limits their ability to manipulate small objects.

There were at least two major groups of australopithecine, one with very large teeth and heavy jaw muscles referred to as robust, and another referred to as gracile. The main difference was in the size of the jaws and teeth. Beyond that, there was no appreciable difference in body size. The evidence
suggests that the large-toothed robust group ate primarily plant foods, where as the gracile group concentrated on a more diverse diet that included meat. Details known about each group are delineated below.

Early Australopiths or Gracile Group

Ardipithecus ramidus. Discovered in 1994 and estimated at 4.4 million years old. This ancient line suggests a close relationship with apes and chimps because of the enamel found on the teeth. Whether or not it walked upright is unknown.

Australopithecus anamensis. Discovered in 1995 and estimated at four million years old. Jaws were apelike but the legs were humanlike; it may have walked upright.

Australopithecus afarensis. Discovered in 1974 by Donald Johanson and known as "Lucy." Estimated at 3.9 to 3.1 million years old. Thought to walk upright and bipedal, these may have left footprints in volcanic ash in Laetoli 3.7 million years ago. Fossils show sexual differences, and suggest that they were adept at climbing trees.

Australopithecus africanus. First found in 1924 by Raymond Dart, this was the first known australopith. Dating from 3 to 2.4 million years ago, it had forelimbs longer than legs and walked upright. Many feel this is the best candidate as ancestor to early Homo species.

Later Australopiths or Robust Group

Australopithecus aethiopicus. Found in 1985, this group dates from 2.7 million years ago. The skull, known as "the black skull," shows a possible relationship with A. afarensis.

Australopithecus boisei. This group lived over a long period of time, between 2.3 and 1.2 million years ago. This skull has the most specialized features of the robust group, with a massive, wide face capable of withstanding extreme chewing forces.

Australopithecus robustus. This group lived between 1.8 and 1.3 million years ago in the same region as A. africanus. This group had jaws, teeth, and habitat similar to A. boisei, but the groups appear to not be related.

Evolution of Modern Humans

Homo habilis.

After researchers unearthed the australopithecines, the next major "missing link" to be found was Homo habilis, an early representative of modern humankind. Found by Louis and Mary Leakey at Olduvai Gorge in Tanzania, these fossils date to between 2.5 and 1.7 million years ago. This creature was bipedal, fully upright, and had the capacity to use forearms for handling tools and weapons.

These fossil specimens show an increased brain size of 600 cubic centimeters (37 cubic inches), and a jaw and tooth size more closely resembling modern humans. Any residual physical traits for climbing had also disappeared. Cut marks on bones suggest the use of tools to prepare meat. They probably retained some of the skeletal characteristics of the australopithecines that made them great climbers. They may have spent considerable
time in trees foraging, sleeping, and avoiding predators. They were the first of our relatives to have opposable thumbs, and the fossil skulls show physical traces of asymmetrical brain development, which is reflected in the way that stone tools were shaped.

Some researchers feel that Homo habilis had a large enough brain to have the rudimentary capacity for speech that may have encouraged cooperation and sharing amongst members of a group. That our distant H. habilis ancestors were able to produce such tools demonstrates that they had manual dexterity but also a capacity for planning, as well as knowledge about what kinds of stones to use and where to find them. The technology of these first toolmakers existed for more than 800,000 years.

Homo ergaster and Homo erectus.

Next in the story of human evolution, we find a group represented by Homo ergastor, a recently recognized African link between Homo habilis and Homo erectus. This group lived from about 1.8 million to 1 million years ago, when Homo erectus and other forms replaced it. Homo erectus fossils found in Java and the Republic of Georgia at 1.9 million years old and 1.6 million years old, respectively, indicate an early migration of Homo ergaster from Africa followed by Homo erectus evolving in Asia and spreading to other areas.

A fossil skeleton of Homo ergaster found in Kenya in 1984 became popularly known as Turkana Boy. This skull led researchers to believe this group may have been the first "naked ape." This specimen suggested no body fur, a dark pigmented skin, and no evidence of living in trees. This species may have reached up to 1.8 meters (6 feet) in height; they appear to have had a near modern size brain and a striding gait. They may have been the first to make and wear clothing of some kind.

Homo ergaster made stone tools, including well-made hand axes and cleavers for the butchering and processing of hunted animals. This technology appeared in Africa and was later carried into western Asia and Europe by Homo ergaster or its descendants. This technology was widespread and used until the end of the Early Stone Age, only a few hundred thousand years ago.

It now appears certain that Homo ergaster was the direct ancestor to the first inhabitants of Eurasia, including Homo erectus in the Far East, as well as the predecessor of Homo sapiens and Homo neanderthalensis in Europe. Homo ergaster led to Homo erectus, the famous missing link, which is our first ancestor to occupy territory from what is now northern China in Asia, to southern Great Britain and Spain in Europe, and all of Africa.

Emergence of Modern Human Beings

Neanderthals and Modern Homo sapiens.

The origin of modern humans is still controversial. The debate centers on whether modern humans have a direct relationship with Homo erectus or the Neanderthals, a well-known, more modern group of humans who evolved within the last 300,000 years. Some researchers feel that modern humans originated separately in Asia, Europe, and Africa. Others feel that modern humans originated in Africa and after migrating into Europe and Asia they replaced the Neanderthals or archaic Homo sapiens found there.

For many years, scientists believed that Neanderthals were the direct descendants of modern humankind. In the 1960s an interesting theory proposed
that different groups of Homo ergaster gave rise to numerous groups of Homo sapiens, including a group known as Neanderthals. This theory suggested that the Neanderthals had disappeared because of being outcompeted by and having interbred with Homo sapiens sapiens, sometimes referred to as Cro-Magnon people. However, more recent evidence suggests a different story.

In a landmark study conducted in 1997, scientists examined the mitochondrial DNA of a Neanderthal fossil and a modern human. This analysis done by molecular biologists provides evidence about when two populations of people last had a common ancestor. The results concluded that it is unlikely that Neanderthals were related to modern humans. Instead it is thought that Neanderthals were a distinct species that evolved side-by-side with early Homo sapiens for hundreds of thousands of years. In addition, the earliest version of Homo sapiens, one with the characteristics that would link it with a common ancestor for Neanderthals and modern humans, Homo sapiens sapiens, had evolved in Africa from Homo ergastor at least 600,000 years ago.

The scientists further calculated that, while Neanderthals and modern humans did indeed share a common ancestor, Homo ergastor, the two lineages had diverged sometime between 550,000 and 690,000 years ago. This established that the Neanderthals evolved in Europe and evolved from archaic Homo sapiens and go back perhaps nearly 300,000 years. It appears that the Neanderthals almost made it to the present, as they appear to have died out only 30,000-40,000 years ago, for reasons not fully understood at this time.

Compared with Homo sapiens, Homo neanderthalensis was barrel-chested with massive brow ridges, a nose that protruded forward, a low sloping forehead, a lower jaw without much of a chin, thick arm and leg bones, and heavier muscles in the shoulder and neck. The brain was actually larger than that of modern humans, possibly because of the additional capacity needed to control the extra musculature. Although the brain size of Neanderthals overlapped with early and modern Homo sapiens, the shape of the cranium was different, suggesting that perhaps the frontal cortex, which controls "higher thought," was restricted.

The Neanderthals also appear to have been culturally quite advanced. While most lived in caves, it is possible that some may have begun to build house-like structures. They manufactured a variety of stone tools, including spear points, scrapers, and knives. They used and controlled fire, which probably helped in cooking frozen meat and in keeping warm. Evidence that they buried their dead with flowers and tools suggests that perhaps they had a form of religion.

It appears, then, that modern human beings are direct descendants of a group known as Cro-Magnon Homo sapiens that appeared in Europe and Asia 100,000 years ago. Although they overlapped with the Neanderthals, they were physically unrelated. They appeared thoroughly modern, with a high forehead and a well-defined chin. Artifacts and stone tools demonstrate that they had mastered the art of making tools and useful instruments from stone, bone, and ivory, and they may have used spears. A number of colored paintings left on cave walls suggest an evolving, rich, and complex cultural life. They hunted cooperatively and were perhaps the first to have a language.

Conclusion

In March 2001 a new fossil, known as Kenyanthropus platyops, was added to the family tree of early humans. Thought to be 3.5 million years old, it is considered a new genus and species of an early human ancestor that lived in the same area and time of Lucy. This recent find is an example of the ways in which our long and complex past slowly reveals itself to us as we come to recognize and understand more about our human ancestors, piece by piece and fossil by fossil.

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Evolution, Human

Encyclopedia of Science and Religion
COPYRIGHT 2003 The Gale Group Inc.

Evolution, Human

Human evolution is a field of science that falls within the larger area of physical anthropology. Human evolutionary studies are broadly synonymous with paleoanthropology, although paleoanthropology is a slightly wider concept that covers the host of fields contributing to the understanding of the human biological past in all its varied aspects. The central concern of human evolution involves sorting anatomical and behavioral differences within and among hominid species in order to delineate their ranges of variation through geological time and across geographical space. Hominid is often used as a colloquial term to indicate membership of fossil forms in the family Hominidae, the taxonomic group that includes anatomically and behaviorally modern humans and their precursors of the last six million years. The term human is a more subjective notion, whose limits can be debated. Some writers use it to include all members of the hominid family, while others restrict it to the genus Homo or to the species Homo sapiens.

In pre-evolutionary times, the Swedish naturalist Carolus Linnaeus (1707–1778), in his first edition of the Systema Naturae (1735), classified all organic organisms into a natural order using a hierarchical system with binominal nomenclature. He included humans (under the genus Homo and the species sapiens, derived from the Latin words for "man the wise"), along with lemurs, monkeys, and apes, in the order Primates. Intriguingly, in place of supplying physical characteristics to define this new species, Linnaeus avoided controversy by simply writing nosce te ipsum ("know thyself"). More than two and a half centuries later, physical anthropologists are still unable to agree on what constitutes modern humanity.

In terms of the morphological definition of modern humans, only a small number of unique anatomical characteristics stand out: (1) Homo sapiens is the only surviving member of the family Hominidae, a group anatomically committed to terrestrial bipedalism; (2) Members of this species have (not uniquely) relatively large brains—averaging 1,350 milliliters—with the most complex neocortex of all primates; (3) their chin-bearing faces are small compared to their neurocrania; and (4) they have a brow region structured into two parts. Behaviorally, modern humans are identified by the unique presence of: (1) a spoken language; (2) the cognitive faculties to generate mental symbols, as expressed in art; (3) the ability to think, reason, and plan; and (4) a bizarre inability to sustain prolonged bouts of boredom. Are anatomically modern humans and behaviorally modern humans the same thing? Not entirely. Anatomically and behaviorally modern humans appear in the archaeological and fossil records at different times.

Approximately one hundred thousand years ago, or perhaps somewhat earlier, anatomically modern humans appear in the fossil records of the Middle East and Africa; they are similar both cranially and postcranially to modern humans today, yet these earliest forms left no archaeological evidence to lead us to believe they had incorporated a modern behavioral repertoire. At seventy to fifty thousand years ago, we detect no change in the morphology of early anatomically modern humans, but there is dramatic evidence of a change in behavior. Splendid murals painted on the walls and ceilings of caves, musical instruments, and elaborate notations, together with a complex technology of stone and bone, are known from western Europe beginning about thirty thousand years ago. But these dramatic expressions were rather late, compared to the suggestions of similar symbolic behaviors known from as long ago as seventy thousand years, and maybe even more, in Africa. Similarly, modern humans had arrived in Australia by sixty thousand years ago, and an effectively modern level of cognition must have been present in these people to have allowed them to cross at least fifty miles of open ocean to get there. Obviously, a cognitive gulf was breached at some time after about seventy thousand years ago (perhaps earlier). This arose first of all in Africa, and spread thence to other parts of the world. Once Homo sapiens was in this behavioral mode, the speed of technological and other behavioral innovation (formerly episodic and rare) increased out of all proportion to what had gone before. At what point religious awareness was acquired is not known, but it was probably part of an overall biological potential for modern cognition that was achieved as a single "package." The huge range of behaviors made possible by this potential was only gradually discovered—and indeed, Homo sapiens is still enlarging its behavioral range today.

The human species and religious doctrine

By nature humans are inquisitive beings with an unquenchable thirst to understand and explain the meaning of life, especially their own. Since the days of the ancient Greek philosopher Aristotle (384–322 b.c.e.), the organic world had been looked upon as stable and unchanging, ascending steadily from the simplest forms to the most complex. Under the doctrine of the "Great Chain of Being," humans were perceived as godly creations and were positioned just below the angels on the top branch of a nicely ordered tree of life. All flora and fauna were designed for the purposes in nature that they were perceived to fulfill. The humanistic ideas of the Renaissance period centered all philosophy on human values and exalted human autonomy and superiority to the rest of nature. By the late seventeenth century, René Descartes's (1596–1650) philosophical idea that animals were complex machines with no higher sense of purpose had been expanded by French and German philosophers to create new foundations for a human social order. Morality was no longer considered to descend from an absolute truth enshrined in Christian beliefs, nor was the notion of accountability in the afterlife. The study of human nature became the key to understanding moral order in decent, complex societies. At a later period some struggled to integrate humans and nature with materialistic philosophy, but this view lost support during the turmoil of the French Revolution.

From Cuvier to Darwin

It would not be until the eighteenth century that the study of human origins became an approachable, but still controversial, topic within the budding science of natural history. Doubts raised by some natural historians questioned the interpretations of biblical literalists as to how humans came to exist on Earth, especially as increasing fossil discoveries in recognizably ancient sediments came to reveal that Earth's fauna did indeed appear to have a biological past. It was evident to naturalists that the Earth bore scars of an ancient history that contained puzzling geological phenomena, such as fossil fish on the tops of mountains, that were inexplicable within the boundaries even of the rudimentary scientific understanding that then existed.

It was impossible, then, to avoid the question as to where humans fitted into the picture. In 1830, the French naturalist Georges Cuvier's (1769–1832) treatise on fossil fauna and flora that was discovered in ancient geologic strata reported no evidence of human fossils coeval with these ancient genera. Since the geologic deposits involved varied greatly from one layer to the next, with bony evidence of once living creatures present in places where they had either gone extinct or now existed only on other continents, Cuvier reasoned that divinely instigated catastrophes and re-creations were responsible for the many extinction and replacement events he perceived. He argued that human fossils could be found if one were to look under the deepest of oceans, as suggested by the Old Testament's story of the great flood. Other naturalists, like Etienne Geoffroy Saint-Hilaire (1772–1844) and Jean Baptiste de Lamarck (1744–1829), provided strict evolutionary reasons for the drastic changes observed in the fossil record. Lamarck, for example, postulated that anatomical and behavioral changes acquired in a creature's lifetime might be passed on to its descendants. However, the Lamarckian paradigm of evolution would shift when two important events took place: (1) the 1858 announcement of Charles Darwin's (1809–1882) and Alfred Russel Wallace's (1823–1913) mechanism of natural selection to explain how species gradually change over time; and (2) the 1856 discovery (and the 1864 naming) of an extinct human species.

Charles Darwin, who rejected the basic tenets of the inheritance of acquired characteristics, enormously popularized a different evolutionary explanation for life on Earth with his the On the Origin of Species, published in 1859. Darwin proposed that biological organisms gradually evolve over time by adapting to their environments. Those individuals who are optimally suited to their environments end up producing more descendants than those who are not. If the features that make them better "adapted" are passed along by biological inheritance to their offspring, those features will become more common in the population, whose aspect will thus change over time. Keenly aware of the controversy it would generate, the retiring Darwin minimized any reference to humans in his publication, and did not broach the problem of human origins until many years later. Darwin's theory of "descent with modification" generated a great deal of controversy within religious and scientific communities. The highly public and politico-religious uproar that resulted centered on the distasteful suggestion that humans and apes share a common ancestor, especially in view of the long held belief that other animals are unable to think and are effectively nothing more than soulless automatons. Coming to Darwin's defense, Thomas Henry Huxley (1825–1895) fervently defended the tenets of Darwinian evolution, most publicly in his debate with Bishop Samuel Wilber-force (1805–1873) in 1860. In his influential 1863 publication of a series of public lectures titled Evidence as to Man's Place in Nature, Huxley argued that humans should be seen as biological organisms, and subject to the same natural laws that all other organic entities obey.

Interpreting the hominid fossils

The second epochal event for human evolutionary studies was the 1856 discovery of a fossil human at the Feldhofer Grotto in the Neander Valley, Germany. Most authorities of the day dismissed this find as the remains of a "barbarous" type of Homo sapiens. However, in 1864 the anatomist William King named the new form Homo neanderthalensis, thereby implying that there had been at least one ancient human extinction and speciation event. With further discoveries of the remains of extinct fossil humans, evolutionary concepts were more palatably applied to modern humans. The British geologist Charles Lyell (1797–1875), once a firm believer in God's role, abandoned many of his theological notions and accepted Darwin's theory of descent with modification after examining the remains of the Feldhofer Neanderthal.

At the turn of the twentieth century, the rediscovery of Mendelian genetics provided a basis for Darwin's evolutionary mechanism. Nonetheless, some paleontologists continued the attempt to integrate Christian beliefs with the idea of evolution. One such was the French Jesuit Pierre Teilhard de Chardin (1881–1955). While in Jesuit training in England, Teilhard also trained in paleontology and archaeology, and became embroiled in the Piltdown controversy that was just erupting. In 1912, he was invited to the Piltdown site in Sussex, which had yielded fossil bones including those of a human, and flint tools. Upon arrival he found a tooth. Reconstruction of the fragmentary hominid pieces seemingly offered the perfect transitional candidate from apes to humans—perhaps too perfect.

In 1912, "Piltdown Man" was introduced to the world as Eoanthropus dawsoni. At that time, the large brain was considered to be the hallmark of humanity; and for forty years British anatomists would disregard many significant fossil human discoveries because of their prized and large-brained Piltdown fossil. Teilhard later continued his paleontological research at the "Peking Man" site of Zhoukoudian in China. The Chinese fossils helped Teilhard to reconcile his now expansive knowledge of the human fossil record with his Christian beliefs. In The Phenomenon of Man (1938–1940), Teilhard proposed a theory of human evolution in which humans were evolving towards a final spiritual unity, also known as Finalism. This notion elicited the disapproval of his Jesuit superiors.

Early in the 1950s, Piltdown was exposed as a hoax—the doctored remains of a human and orangutan—and Teilhard has even been fingered as the hoaxer, though he remains only one of the more unlikely suspects of many. By the late 1950s the human fossil record had greatly expanded, as had the plethora of names used to describe it. A tidying-up was in order, and this was gradually achieved under a gradualist and progressivist model of human evolution.

In the 1970s and 1980s, new systematic methods began to transform the understanding of the constantly expanding human fossil record. Further, molecular studies were providing new perspectives. In particular, the "molecular clock" shortened the ape-human divergence to as little as five to six million years ago (from maybe twelve to fourteen). From around 1970 researchers uncovered bipedal but otherwise rather apelike hominids from sites in eastern Africa. These joined the Australopithecus fossils already known from southern Africa in the 2.5 to 1.5 million years ago range, and dated mostly from about 3.5 to 2.0 million years ago. Interpreted using an underlying gradualist model, these archaically-proportioned fossil hominids mostly reflected the search for an "earliest ancestor."

The situation at the beginning of the twenty-first century

Over the following few decades, hundreds of fossil human discoveries offered fuel for systematic debates. The "single species hypothesis," which stated that the human ecological niche was so wide that only one species of hominid could have existed at any one time, was rapidly invalidated by new finds, but still lingers in models of human origins that find deep roots in time for contemporary geographical groups of humankind. Evolutionary theory, as well as the rather sparse fossil record, imply in contrast that the species Homo sapiens must have had a single origin at one time and in one place, probably Africa. All of the human diversity familiar today has apparently appeared within the past 150 thousand years or so.

Despite minor differences of opinion, it is clear that the diversifying pattern of human evolution is similar to that of other mammalian taxa. Hominid phylogeny is a story of evolutionary experimentation, with multiple speciations and extinctions. The hominid family comprises at least five genera and eighteen known species (see Fig. 1, p. 302), some of which shared territories in both time and space. At present, all geographical varieties of modern humans occupy the single surviving twig of what appears once to have been a densely branching bush.

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Human Evolution

Biology
COPYRIGHT 2002 The Gale Group Inc.

Human Evolution

It is remarkable that the unique qualities of humans—language, advanced thought, and culture—evolved through the same processes that shaped the adaptations of all other creatures on earth: natural selection. How this came
about is perhaps the most fascinating question of all time. The direct evidence for human evolution has increased enormously since the early 1990s via the discovery of hundreds of new fossils, including three new genera and even more new species; and via the comparisons of modern and ancient deoxyribonucleic acid (DNA). Great leaps forward are being made in science because of this proliferation of information.

Humans and our ancestors are called hominins, going back to the time of the split from the lineage of human's closest relatives, the chimpanzees. (Until recently the term hominid was used, but based on genetic relatedness and the rules of zoological nomenclature, the word hominid should apply to both the chimpanzee and human clades [lineages].) Humans have evolved various traits that have diverged from the typical great ape collection of characters; notable is human's commitment to standing on two legs (bipedalism) rather than four, increased fine motor dexterity making possible extensive use of tools, prolonged period of infancy and childhood, increased brain size, language, great cultural complexity and economic interdependence.

Ape Ancestors

During the Miocene epoch (23 to 5 million years ago), when our super-family, the Hominoidea (apes) flourished and speciated, more than twenty genera and about twice that number of species in the Hominoidea family. Many of these species went extinct, and today there are only five genera of apes: gibbons, orangutans, gorillas, chimpanzees/bonobos, and humans. The
origins of the hominins remain obscure. Genetic comparisons and the molecular clock suggest that the human-chimp lines split in the late Miocene about 6 million years ago. The fossil record is very incomplete at this time, and neither fossils of ancestral chimpanzees nor fossils that are clearly ancestral to both lines are known. However, field research since the mid-1990s has resulted in several fossils from this time period, and researchers hope that this trend of discovery will continue and scientists' understanding of the origin of hominins will improve over the twenty-first century.

Scientists do know this: Hominins evolved from an ape that was similar to the living African apes, and the human lineage did not spread beyond Africa into Eurasia until about 1.8 million years ago. The early hominins lived in an array of habitats, but most evidence points to wooded savannas (a type of grassland) as a principal one. There is solid evidence that significant amounts of bipedal behavior preceded other major events such as increased brain size or tool use.

Earliest Hominins

The earliest named hominin, Orrorin tugenensis, was discovered in 2000 in eastern Kenya, and is believed to have lived 6 million years ago. Thigh bones suggest that this species spent a significant amount of time bipedally. In the 5-million-year time range there are several fragmentary fossils from East Africa, but they are difficult to associate with particular species. At 4.4 million years ago, another genus and species, Ardipithecus ramidus, is present. This species has many traits, especially in the teeth, that are between chimpanzees and later hominins (australopithecines) in size and character (for example canine size, enamel thickness, and size of the cheek teeth). Discovery of a third new early hominin, Kenyanthropus platyops, was published in 2001 by Meave Leakey and colleagues.

The newly discovered Kenyanthropus platyops dates from the same era as the Australopithecus but belongs to a previously unknown species. The new skull has a more humanlike face with much smaller teeth. Its name means "the flat-faced human from Kenya."

Between 3.9 and 2.0 million years ago there was a proliferation of australopithecine species, including the "gracile" australopithecines of the genus Australopithecus, with five commonly recognized species, and the robust australopithecines of the genus Paranthropus, with three commonly recognized species. These australopithecine species are variable, but general trends include: significant amounts of terrestrial bipedalism, with some species also retaining significant arboreal adaptations; brains still very close to chimpanzees' in size; tool use generally minimal; very large grinding cheek teeth and chewing muscles, consistent with a largely vegetarian diet including many tough foods; canine teeth reduced in size; and male body size much larger than females (that is, high degrees of sexual dimorphism).

Origins of Homo

The origins of the human genus Homo is also being reexamined in the early twenty-first century. There are many fossils in the 2.4- to 1.6-million-year time range in Africa that have variously been assigned to Homo habilis, Homo rudolfensis, or Homo species, but many still have large cheek teeth, smallish brains, and faces shaped like australopithecines. Some of these fossils even retain many arboreal adaptations. Thus the majority of these specimens will probably be moved into the genus Australopithecus; reserving the genus Homo to include only those specimens which are clearly more closely related to humans.

Considered this way, the human genus evolved nearly 2 million years ago with the appearance of Homo ergaster. Homo ergaster, personified by the amazingly well-preserved West Turkana skeleton of a twelve-year-old boy, is a markedly different creature than australopithecines, though still not modern. They were tall, lanky, long-legged with body proportions nearly the same as modern humans, committed to efficient terrestrial bipedalism at the expense of arboreality, and larger—about the size of modern humans. This increase in body size was accompanied by an increase in brain size—the largest examples having cranial capacities (brain cavities) of about 900 cubic centimeters (cc), the smaller examples closer to 650 cc. (Modern human brains are about 1300 cc, chimpanzees are about 400 cc.) Because of the increase in body size, this is not a large relative increase in brain size over earlier hominids. The trend toward increasingly large cheek teeth is reversed in our lineage: There is a reduction in cheek tooth size, generally thought to reflect consumption of higher quality foods and/or more food preparation.

Homo ergaster also used tools. The earliest archaeological sites, which contain the debris of stone tool manufacture and animal bones eaten by hominins, date to 2.6 million years ago in Ethiopia. The earliest stone tools are simply sharp flakes broken off rocks that were used as cutting tools. It is difficult to say which species made and used these earliest tools, in fact it probably included some of the australopithecine species. By 1.5 million years ago the stone tool industries include more elaborate, symmetrically flaked rocks, called Acheulian hand axes.

Spread Beyond Africa

Immediately after the evolution of Homo ergaster there was an initial, but probably short-lived, spread beyond Africa into the Near East, Far East (Java), and southern Spain. Either something about H. ergaster or the environment must have facilitated this spread. Was it the increase in body size? Did their ecological position shift up as they became more successful at acquiring meat, and therefore their home ranges needed to increase to encompass more and more space? Was it learning to control fire? Or changes in social organization? More research will focus on these questions, but whatever caused this initial spread, there is very little evidence for Homo outside of Africa again for about 1 million years, when more sites are found around Europe and Asia.

The populations in the Far East (China and Indonesia) evolved into a local species called Homo erectus, which may have survived until into the Late Pleistocene (approximately 50,000 years ago). These fossils have long low skulls with thick brows and cranial capacities near 1000 cc. Interestingly, H. erectus seems to have been contemporary with other species of archaic Homo, specifically species that evolved into Homo neanderthalensis in Europe and the Near East, and another that evolved into anatomically modern humans (AMH) in Africa. All of these species have long, low skulls, big faces, and large brow ridges, but differ in details of the cranial anatomy, and some have larger cranial capacities. They are robust with massive skeletons and muscles, reflecting a large dependency on brawn to cope with the world. Archaeological evidence for accomplished hunting in the form of wooden spears is clear by 400,000 years ago.

In Europe Neanderthals evolve into their classic form: extremely strong, stocky, and robust, with large eyes and noses and jutting faces; their bodies seem well adapted to the cold Pleistocene. Their tool kit was more elaborate, including stone-tipped spears, and archeological and isotopic evidence indicates a great reliance on meat in the diet. Nonetheless, except in a very few instances when Neanderthals were contemporary with modern humans, there is little to no evidence for art. For this reason, many anthropologists believe Neanderthals did not have modern language.

Anatomically Modern Humans

Meanwhile, archaic Homo evolved into anatomically modern humans in Africa. Throughout this time range (between 200,000 and 100,000 years ago) there are increasing bits of evidence for art and symbolism in the form of red ochre, beads, and composite tools in Africa while they are still absent in Europe, and it is during this period that genetic evidence from modern humans suggests that anatomically modern humans evolved. Indeed, the earliest anatomically modern humans fossils come from Africa and the Near East, just over 100,000 years ago. Anatomically modern humans are characterized by a reduction in skeletal robustness and strength, probably related to greater reliance on technology and culture rather than brute strength. Cranial capacity remains about the same as in the more robust predecessors, but the face and teeth are smaller, the forehead becomes high and the chin juts out. Cultural elaboration is evident in increased number of tools types, regional variation in style, more composite tools, and notably, art.

There is a minority of anthropologists who consider all the Homo specimens to be from one diverse species. In the early twenty-first century, the majority of anthropologists believe that the fossil, archaeological, and genetic evidence concur that anatomically modern humans evolved in Africa about 200,000 to 150,000 years ago, and spread out from there to Europe and Asia as recently 100,000 to 25,000 years ago (depending on where), replacing the archaic Homo species, for example, Neanderthals and Homo erectus, and these later species may have contributed relatively little genetic diversity to the human gene pool. The information scientists have thus suggests that all humans across the globe today are very closely related to one another.

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Human Evolution

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Human evolution

Since the mid-nineteenth century, Western scientific thought has stated that all present-day species on Earth, including man, have arisen from earlier, simpler forms of life. This theory means that the story of human evolution begins with a creature most people today would not consider human.

In 1859, the view of man's history and his place on Earth was changed forever by the publication of On the Origin of Species by Means of Natural Selection, written by English naturalist Charles Darwin (1809–1882). In this revolutionary book, Darwin stated that all living things achieved their present form through a long period of natural changes. In his 1871 book, The Descent of Man, Darwin further argued that man descended from subhuman forms of life.

The history of how the human species evolved has been reconstructed by evidence gathered by paleontologists (who study fossils), anthropologists (who study humans and their origins, development, and customs), anatomists (who study the structure of biological organisms), biochemists (who study chemical compounds and processes occurring in biological organisms), and many other scientists. Most of the concrete evidence comes from the record left by fossils, which are remains or imprints of ancient plants or animals that are found in layers of rock. In practice, human fossils are mostly bones and teeth, which are the parts of the human body more likely to be preserved over a great time period.

The human fossil record

In the overall history of life on Earth, the human species is a very recent product of evolution. There are no humanlike fossils older than 4.4 million years, which makes them only one-thousandth the age of life on Earth.

The human species, or Homo sapiens, belongs to the hominid family tree. Hominid means "human types," and describes early creatures who split off from the apes and took to walking upright or on their hind legs. Studies have shown that Homo sapiens share a clear anatomical and genetic relationship to other primates (members of the group of mammals that includes lemurs, monkeys, apes, and humans). Of all the primates, humans have the closest relationship with apes. Both species have descended from a common ancestor. At some point in evolution, the branch of primates split into two arms. One evolved into modern apes, while the other evolved into modern humans.

The reasons that human ancestors started to walk upright are not known. Possibly, it was a response to environmental changes; as tropical forests were beginning to shrink, walking might have been a better way to cross the grasslands to get to nearby patches of forest for food. Standing upright also may have been a means of defense that slowly evolved. When chimpanzees or gorillas become excited, they stand in an upright posture and shake a stick or throw an object. By standing upright, they appear bigger and more impressive in size than they normally are. In addition, the ability to stand up and get a wider view of the surroundings gives an animal an advantage in the tall grasses. Walking upright also frees up the hands to carry objects, such as tools.

Words to Know

Fossils: Remains or imprints of ancient plants or animals that are found in layers of rock.

Hominid: Member of the family of primates that includes modern humans.

Primate: Member of the group of mammals that includes lemurs, monkeys, apes, and humans.

Australopithecus. One of the oldest known humanlike animals to have walked upright is believed to be Australopithecus afarensis, meaning the southern ape of the Afar region in Ethiopia, Africa, where the fossils were found. The most famous of these fossils, nicknamed Lucy, was found in 1974 near Hadar, Ethiopia, by a team of anthropologists led by American Donald Johanson (1943– ). Lucy lived about 3.18 million years ago, and had a skull, knees, and a pelvis more similar to humans than to apes. Her brain size was about one-third that of modern humans, yet larger than any apelike ancestor to have come before. She would have stood about 3.5 feet (1 meter) tall, with long arms, a V-shaped jaw, and a large projecting face.

In 1924, Australian anthropologist Raymond Dart (1893–1988) discovered fossils at a site called Taung in South Africa. These fossils, dated at 3 million years old, were named Australopithecus africanus, meaning the southern ape of Africa. Australopithecus africanus probably evolved from Australopithecus afarensis, but was slightly taller and had a slightly larger brain. Altogether, there were probably four main species of australopithecines.

Kenyanthropus. In early 2001, discoverers and scientists of human evolution were stunned by the announcement of the discovery of a 3.5 million-year-old skull from what appeared to be an entirely new branch of the early human family tree. The skull was discovered in 1999 by a research team led by Meave Leakey on the western side of Lake Turkana in northern Kenya. Leakey named the new member of the hominid family Kenyanthropus platyops, meaning flat-faced man of Kenya. With a flattened face and small molars, this hominid differed significantly from the contemporary species to which Lucy belonged. This discovery has led paleontologists to theorize that the human family tree is not one with a straight trunk, but one shaped more like a complex bush with a tangle of branches leading in many directions. Some branches lead to other branches, while some lead to dead ends.

Homo. From one of these previous branches came the oldest known hominid given the Latin name Homo, or "man." This was Homo habilis, or "handy man." Discovered by English archaeologist and anthropologist Louis S. B. Leakey (1903–1972) in 1961 in Olduvai Gorge, Tanzania, this hominid was present in east Africa at least 2 million years ago. Taller than his predecessors, Homo habilis showed the first marked expansion of the brain. He was the first hominid to use tools routinely.

By about 1.6 million years ago, the hominid brain had increased to about one-half the size of what it is today, and this difference made for a new classification, Homo erectus, or "upright man." Homo erectus is generally thought to have been modern man's direct ancestor. The first known fossil of Homo erectus was found by Dutch paleontologist Eugène Dubois (1858–1940) in 1894 in Java (an island of Indonesia); it was nicknamed Java man. Homo erectus is believed to have lived between 250,000 and 1.6 million years ago, although recent scientific findings on Java indicate that Homo erectus may have lived there until about 27,000 to 53,000 years ago. Homo erectus was the first hominid to use fire and hand axes.

Anthropologists have long agreed that the first humans arose in Africa. Just when these early humans began to migrate out of Africa and inhabit other continents, however, has been a matter of fierce debate. But

in May 2000, a team of anthropologists reported the first undisputed proof that humans indeed left Africa at least 1.7 million years ago. The team found two well-preserved skulls about 53 miles (85 kilometers) south of Tbilisi, the capital of the Asian nation of Georgia. The skulls closely resembled those of an early human species—called Homo ergaster by some scientists and early Homo erectus by others—known to have lived in Africa between 1.9 million and 1.4 million years ago. Scientists say the find demonstrates that Homo ergaster was on the move shortly after this new species arose in Africa and that some of our earliest ancestors were already restless wanderers.

Homo sapiens. Between 250,000 and 400,000 years ago, Homo erectus evolved into Homo sapiens ("wise man"). These ancestors of modern man cooked their food, wore clothing, buried their dead, and constructed shelters, but did not have a modern-sized brain. Over time, the body and brain of Homo sapiens gradually became somewhat larger.

By about 40,000 years ago, Homo sapiens had evolved into modern human beings, Homo sapiens sapiens ("wise, wise man"). In 1868, the first fossils of modern Homo sapiens sapiens were found in Cro-Magnon caves in southwest France, which gave that name to all early Homo sapiens sapiens. Cro-Magnon remains have been found along with the skeletons of woolly mammoth, bison, and reindeer and with tools made from bone, antler, ivory, stone, and wood, indicating that Cro-Magnon hunted game of all sizes. Cro-Magnon buried their dead with body ornaments such as necklaces, beaded clothing, and bracelets.

Cro-Magnon were artists, producing hauntingly beautiful cave art. Carefully rendered pictures of animals, human and mythical representations, and geometric shapes and symbols were created using charcoal and other pigments. Carvings of stone, ivory, and bone have also been discovered in these caves.

In the late twentieth century, new fossil discoveries and genetic evidence fueled a debate concerning when and where Homo sapiens sapiens emerged. In 1988, researchers found fossil fragments in a cave in Israel that suggest that anatomically modern humans lived there 92,000 years ago. These findings support the theory that modern humans existed much longer than previously believed. They also support the theory, called the out-of-Africa model, that modern humans evolved only once, in Africa, leaving there in a rapid global expansion to replace other populations of older human forms in Europe and Asia. The out-of-Africa model is opposed by the multiregional model, which argues that modern humans arose almost simultaneously and independently in several different places in Africa, Europe, and Asia.

Neanderthal man

Neanderthal man (Homo sapiens neanderthalensis ) was the first human fossil to be found in modern times. It was discovered in 1856 in Germany's Neander Valley. These early humanlike hominids (the source of the caveman stereotype) had a large brain, a strong upper body, a bulbous nose, and a prominent brow ridge. They were proficient hunters. It is possible that Neanderthals had an elaborate culture, were aware of the medicinal properties of plants, and ritually buried their dead. Neanderthals first appeared 300,000 years ago in what is now Europe, lived throughout the ice ages, and disappeared about 35,000 years ago.

Recent excavations in Israel show that Neanderthals were contemporary with modern Homo sapiens sapiens. The two hominids apparently

survived independently of each other for thousands of years. In 1997, a team of German biologists analyzed the DNA (deoxyribonucleic acid; genetic material) extracted from the bones of a Neanderthal who lived at least 30,000 years ago. Their findings indicated that Neanderthals did not interbreed with modern humans. The findings also suggested that the Neanderthal line is four times older than the modern human line, meaning Neanderthals split off much earlier from the hominid line than did the ancestors of modern humans. Scientists do not know why Neanderthals died out, nor what the nature of their interaction with Homo sapiens sapiens might have been.

[See alsoEvolution; Fossil and fossilization; Genetics; Primates ]

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evolution, human

evolution, human The human body is the end product of a long period of evolution, stretching back millions of years. In the case of some aspects of our body, the ancestry goes back not just a few million years, but hundreds of millions. The basic layout of the human body, for example, is that of the vertebrates (being bilaterally symmetrical, organized around the backbone) and of the reptiles and amphibians (in having a pair of hind limbs and forelimbs, each with five digits — fingers or toes — at the end of them). But like every species, humans have a shape that is unique to themselves. Among primates, our closest evolutionary relatives, humans have three features that stand out — upright posture and walking, a relatively large brain, and relative hairlessness.

Most primates live in trees, and they do so like all mammals by using all four limbs (or in the case of spider monkeys, five — their tails are also prehensile and can grasp things). Their hands and feet can both be used for grasping. In this sense, all non-human primates are quadrupedal. In the case of something like an orang utan, legs and arms, hands and feet are equally mobile and dextrous, and in a way all act more like arms than legs — for holding and grasping, rather than support. With baboons the forelimbs and hindlimbs are both rather leg-like, and support the animal as it moves quadrupedally over the ground, rather like a dog. For the gibbon, the only truly arm-swinging primate, the arms are long and flexible, and the legs, short and reduced — basically to get them out of the way as the owner brachiates through the trees.

Everything about the human body is either a retention of these basic characteristics, or else has been modified by evolution. The grasping hand, the relatively mobile shoulder, the eyes that look forward with stereoscopic vision, are all part of the human being's ancient primate heritage. Each evolved for some reason in our past, long before any movement towards the human condition, but has remained useful and has been built upon. The close-set eyes that look directly forward, with overlapping fields of vision, evolved among the earlier primates, to allow them to judge distances in three-dimensional space — an essential part of leaping perilously from one tree branch to another. The ability to co-ordinate this vision with dextrous hand movements is an old evolutionary heritage, but one that is used every time we catch a ball or calculate whether it is safe to overtake a car at 100 km per hour.

While our body is a cumulative and often messy mix of this ancient past, it is also the product of a unique evolutionary history shared with no other living primate. It is often said that humans are the most generalized of species, lacking all the specializations that characterize other animals such as giraffes, with their long necks, or elephants, with their trunks. In actual fact, as primates we are very specialized in one way — bipedalism. Unlike virtually all other primates, we are highly dedicated ground-dwellers, and indeed are fairly poor at climbing and clambering in trees. Our ability to walk upright habitually and easily is our most distinctive and in many ways most divergent characteristic. It has also shaped virtually all aspects of our body, from head to toe. Our foot is effectively a highly sprung platform, with arches in two directions to take the endless pounding of hitting the ground, and to push off into the next stride. It is heavily built compared with the feet of monkeys and apes, and has lost any ability to grasp. The knee is also built to take pressure, being large, and heavily constrained in sideways movement. The leg as a whole is very long, to ensure a large stride. The pelvis is perhaps the most modified part of the body, being turned from a long baton for connecting upper and lower parts of the body, to a large bowl to take all the weight of the upper body, which is now resting entirely on two legs. The vertebral column is also robust. Unlike the back of a quadruped, which is built with a single arch like a cantilevered bridge, the human spine is S-shaped. The head is also modified, being perched more vertically on the spine.

The overall impression of a human from an evolutionary perspective is a tall, cylindical shape, a linear design. There has been considerable debate as to the evolutionary pressures that have shaped the human body, and it looks as if there are two main factors involved. The first is that bipedalism is an energy-saving way of moving on the ground: since our ancestors had to cope with the disappearance of forests, and search widely for food in dry African environments, it was the most evolutionarily effective way, turning an arm-swinging, tree-dwelling ape into a terrestrial specialist. The other factor is temperature. The open savannas where the earliest bipeds evolved were hot, with little shade, and the effect of the sun would have been severe. One of the effects of an upright posture is to reduce the area of the body that receives direct sunlight, and to remove more of it away from the reflected heat of the ground. The human body, then, was forged by selection in the heat of the more open plains of Africa.

Evolution is the process of change over time, over thousands and millions of years. The fossil record has shown that the basics of bipedalism go right back to the roots of our evolutionary history, back to over four million years ago, soon (in evolutionary terms) after our ancestors diverged from the ancestors of the living chimpanzees, our closest relatives. The modern form of bipedalism, with the cylindrical, linear pattern, is probably about two million years old. With bipedalism would have come other changes. The hand, no longer needed to support the body in movement, became the highly dextrous and finely-tuned structure that we use today for so many activities.

The upright stance is such a universal and uniform human characteristic that it is taken totally for granted: it is the essence of humanity. Around the world, though, the human body comes in enormous variety — tall, short, fat, thin, hairy, smooth, dark, and light. Unlike the basic upright body plan, these variations are not millions of years old, but just a few tens of thousands or even less. But they are still the product of evolution. Once again the environment has played a major part. Although humans vary in the amount of hair cover they have, they are, by comparison with apes, largely hairless. This is again a response to heat. Humans have evolved a copious sweating system — we use the evaporation of moisture from the skin to cool our body, and this works more effectively where the air can move freely over the skin — that is, where there is no hair. As a whole, therefore, the species is ‘naked’ — not actually hairless, but with a miniaturized hair cover. And those people who have a long history of living in the hotter parts of the world are the most hairless. Skin colour follows this pattern, with darker skins, produced by higher levels of melanin, acting as a compensatory mechanism to reduce the effect of high levels of solar radiation on the skin. Body shape is also affected by the environment — larger, shorter-limbed bodies are better at keeping in heat, where thin, long-limbed individuals are better at dissipating heat. As a result, people who live at higher latitude have shorter limbs, and are often robustly built; people in the tropics are small, linear, and lean.

While the human body has evolved to suit the environment, especially the temperature, it has been affected by one other major factor — sex. Evolution is driven by selection — the survival of those best suited to the environment — but Darwin pointed out that there were two elements to this; natural selection and sexual selection. Most of the characteristics described so far have been the product of natural selection, but much of the human body is probably the result of how males and females have chosen their mates, and how well they are able to reproduce. Out of this has arisen the differences between the sexes. Some of these differences have a direct function — women have wider hips than men, compensating for the narrower birth outlet forced by bipedalism. Others are probably related to the preferences of men or women — larger breasts and curvaceous hips in women, for example. These secondary sexual characteristics may have their basis in some function, but are as much a signal and a symbol, and selected as such — in this case, a signal of fertility. Men also give signals with their bodies — simple ones related to strength and size, but also more subtle ones, such as grey hair or baldness as a sign of having lived a long time — and therefore being a successful male. Most characteristics, though, are a mixture of the sexual and functional. Men often prefer women who are more curvaceous, which is often related to fat deposition — women lay down fat more easily than men. This fat is also necessary for ensuring that a woman is well-nourished, and thus better able to withstand the costs of pregnancy and lactation. Women may prefer large, strong men, but such men may also be better at other things, such as hunting or fighting, and thus better adapted.

In the end, the evolution of the human body is a seamless mix of sex, reproduction, activity, and environment; it is also a mix of the very old and the very new, and over evolutionary time has changed and shifted. In some ways it is a sleek and efficient machine; in others, it is full of flaws. In this sense it is like any other evolutionary product, a compromise between all the demands placed on it during the course of the many different lives that humans have to live, have lived in the past, and will live in the future.

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human evolution

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

human evolution, theory of the origins of the human species, Homo sapiens. Modern understanding of human origins is derived largely from the findings of paleontology, anthropology, and genetics, and involves the process of natural selection (see Darwinism). Although gaps in the fossil record due to differential preservation prevent the complete specification of the line of human descent, H. sapiens share clear anatomical, genetic, and historic relationships to other primates. Of all primates, humans bear particularly close affinity to other members of a group known as hominoids, or apes, which includes orangutans, gibbons, gorillas, chimpanzees, and humans. Humans and their immediate ancestors, known as hominids, are notable among hominoids for their bipedal locomotion, slow rate of maturation, large brain size, and, at least among the more recent hominids, the development of a relatively sophisticated capacity for language, tool use, and social activity.

The Evolutionary Tree

Humans are mammals of the Primate order. The earliest primates evolved about 65 million years ago in the geological period known as the Paleocene epoch. They were small-brained, arboreal fruit eaters, similar to modern tree shrews. Primates of the Eocene epoch (55 to 38 million years ago) were similar and ancestral to contemporary tarsiers, lemurs, and tree shrews, and are classified as lower primates or prosimians. During the late Eocene, the higher primates, or anthropoids, developed from prosimian ancestors and, aided by continental drift, diverged into New World (or platyrrhine) and Old World (or catarrhine) monkeys. The branching of Old World monkeys and hominoids apparently occurred in the late Oligocene (38 to 25 million years ago) or early Miocene (25 to 8 million years ago), a time period poorly represented in the fossil record. The lesser apes (gibbons and siamangs) and other hominoid lines diverged about 20 million years ago, while the Asian great apes (the orangutan being the only surviving form) diverged from the African hominoids about 15 to 10 million years ago. Genetic evidence suggests that the ancestral lines of gorillas diverged about 8 million years ago and that chimpanzees and hominids diverged about 5 million years ago.

Hominid Evolution

The earliest known hominids are members of the genus Australopithecus, the earliest of which date to more than 4 million years ago. Unlike other primates, but like all hominids, australopithecines were bipedal. Their crania, however, were small and apelike, with an average cranial capacity of about 450 cc in the gracile species and 600 cc in the robust forms. Australopithecines that have been considered ancestral in the lineage leading to the human genus Homo include A. afarensis (an important skeleton of which is popularly known as Lucy) and A. africanus. The exact position of these and other early species on the hominid family tree continues to be disputed.

The first member of the genus Homo, a small gracile species known as H. habilis, was present in east Africa at least 2 million years ago. H. habilis was the first hominid to exhibit the marked expansion of the brain (with an average cranial capacity of about 750 cc) that would become a hallmark of subsequent hominid evolutionary history. By about 1.6 million years ago, H. habilis had evolved into a larger, more robust, and larger-brained species known as Homo erectus (African members of the species are sometimes called H. ergaster). Cranial capacities ranged from about 900 cc in early specimens to 1050 cc in later ones. H. erectus persisted for well over a million years and migrated off the African continent into Asia, Indonesia, and Europe. H. heidelbergensis is believed to arisen from H. erectus as far back as 1.3 million years ago.

Between 400,000 and 350,000 years ago, H. heidelbergensis is believed to have given rise to H. neandertalensis, or Neanderthal man, in Europe, and to an branch in Africa that eventually became H. sapiens. By about 150,000 years ago in Africa and Asia and 40,000 years ago in Europe (see Cro-Magnon man), the transition to H. sapiens was complete, and fully modern humans became the single surviving hominid species (with the possible exception of the humans represented by the remains found on Flores, Indonesia, which may represent a dwarf hominid species that survived until 13,000 years ago).

The Evolution of Culture

Among hominids, a parallel evolutionary process involving increased intelligence and cultural complexity is apparent in the material record. Evidence of greater behavioral flexibility and adaptability presumably reflects the decreased influence of genetically encoded behaviors and the increased importance of learning and social interaction in transmitting and maintaining behavioral adaptations (see culture). Because the organization of neural circuitry is more significant than overall cranial capacity in establishing mental capabilities, direct inferences from the fossil record are likely to be misleading. Contemporary humans, for example, exhibit considerable variability in cranial capacity (1150 cc to 1600 cc), none of which is related to intelligence.

Tool use was once thought to be the hallmark of members of the genus Homo, beginning with H. habilis, but is now known to be common among chimpanzees. The earliest stone tools of the lower Paleolithic, known as Oldowan tools and dating to about 2 to 2.5 million years ago, were once thought to have been manufactured by H. habilis. Recent finds suggest that Oldowan tools may also have been made by robust australopithecines. The simultaneous emergence of H. erectus and the more complex Achuelian tool tradition may indicate shifting adaptations as much as increased intelligence.

While it is clear that H. erectus was much more versatile than any of its predecessors, adapting its technologies and behaviors to diverse environmental conditions, the extent and limitations of its intellectual endowment remain a subject of heated debate. This is also the case for both archaic H. sapiens and Neanderthals, the latter associated with the more sophisticated technologies of the middle Paleolithic. However impressive the achievements of H. erectus and early H. sapiens, most material remains predating 40,000 years ago reflect utilitarian concerns. Nonetheless, there is now scattered African archaeological evidence from before that time (in one case as early as 90,000 years ago) of the production by H. sapiens of beads and other decorative work, perhaps indicating a gradual development of the aesthetic concerns and other symbolic thinking characteristic of later human societies. Whether the emergence of modern H. sapiens corresponds to the explosion of technological innovations and artistic activities associated with Cro-Magnon culture or was a more prolonged process of development is a subject of archaeological debate.

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human evolution

human evolution Process by which humans developed from pre-human ancestors. The fossil record of human ancestors is patchy and unclear. Some scientists believe that our ancestry traces back to one or more species of Australopithicenes that flourished in s and e Africa c.4–1 million years ago. Other scientists believe that we descend from some as yet undiscovered ancestor. The earliest human fossils are those of Homo habilis (handy people) which date from 2 million years ago. The next evolutionary stage was Homo erectus (upright people), who first appeared c.1.5 million years ago. The earliest fossils of our own species, Homo sapiens (wise people), date from c.250,000 years ago. An apparent side-branch, the Neanderthals, Homo sapiens neanderthalensis, existed in Europe and wAsiac.130,000–40,000 years ago. Modern humans, Homo sapiens sapiens, first appeared c.100,000 years ago. All human species apart from Homo sapiens sapiens are now extinct. See also evolution

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